1. Technical Field
This disclosure relates generally to lifting loads using buoyancy-based lifting systems, and more particularly to controlling buoyancy-based lifts.
2. Description of Related Art
The marine salvage industry, off-shore oil & gas industry, nautical archeology, and military operations often use air lift bags to move or recover objects in bodies of water. Two types of lift bags exist: the open-bottom or parachute-type, and the enclosed type. Both types are constructed of flexible fabric and filled with air, another gas, or sometimes another lighter-than-water fluid, to create a buoyant force to lift an object. Rigid, fixed volume devices, such as ballast tanks have also been used for buoyant lifts.
Conventional flexible lift bag systems tend to become unstable during a lift, because the partially-inflated lift bag changes volume inversely with depth, and therefore tends to accelerate upward or downward when displaced even slightly from a position of neutral buoyancy. The risks of instability include diver entanglement in a rapid, uncontrollable ascent or descent, potentially causing injury or death. Also, the lift bag, and load being lifted, may broach the surface at high speed, possibly causing the loss of gas from an open-bottom bag, followed by uncontrolled descent. The high speed ascent or broaching itself may damage the object being lifted. Using lift bags to overcome seabed suction, or to free mechanically-locked or snagged equipment, can also be problematic, as the amount of buoyant force needed to lift the load becomes suddenly less as soon as the load is freed.
In practice, elaborate precautions are often needed in lift-bag operations, such as dead-man anchors and restraining lines, and/or vertically-staged multiple lift-bag arrays which prevent the load from going up in an uncontrolled fashion.
Although rigid volume lifting devices, such as ballast tanks may not suffer from the volume-change instability of flexible lift bags, the rigid volume devices can be excessively cumbersome due to their large size and weight, making them more difficult to transport than flexible lift bags.